Due to limitations in human vision in low light level environments, white light illuminator systems have long been used to produce artificial illumination and enhance visibility during nighttime or overcast conditions or within interior quarters obscured from the reach of solar illumination. Illuminators are therefore generally designed to mimic or reproduce daytime lighting conditions, to the extent possible, so that illuminated subjects of interest are bright enough to be seen and have sufficient visual qualities such as color and contrast to be readily identifiable.
A diversity of illuminator systems such as stationary lamps in buildings, portable flashlights, and vehicular headlamps and courtesy lights have evolved throughout history and have traditionally produced white light for general, spot or flood illumination, using a variety of sources such as candles, oil, kerosene and gas burning elements, incandescent and halogen bulbs, and fluorescent and other arc-discharge lamps. White light is critical in such uses because of its unique ability to properly render colored objects or printed images relative to one another and its similarly unique ability to preserve luminance and color contrast between adjacent objects or printed images having different colors. For instance, a blue photographic image of an ocean panorama will be readily distinguished by an unaided observer from black photographic images of volcanic rocks when the photograph containing these images is illuminated by white light. The two images would, however, be virtually indistinguishable from one another if illuminated with a deeply red colored illuminator. Another example arises from the need to properly identify differently-colored regions on conventional aeronautical or automotive maps. On an automotive map, white light illuminators make it easy to discern the difference between the yellow markings for urban regions and the surrounding white rural areas. A deeply yellow colored illuminator would make this distinction virtually impossible. On an aeronautical chart, white light illuminators make it possible to discern the difference between the characteristic blue markings for certain types of controlled airspace and the green pattern of underlying terrain, whereas a deeply red colored illuminator would make this distinction virtually impossible.
Furthermore, these issues of color discrimination and contrast go beyond the simple need for accurate identification. It is, for example, a well known fact that high contrast is critical for avoiding severe operator eye fatigue and discomfort during prolonged visual tasks, whether the subject of study is a book, magazine, newspaper or a map. White light illuminators provide more universally high contrast and good color discrimination, thereby avoiding these annoying and dangerous physiological side effects.
The extensive evolution and widespread use of white light illuminators, along with rapidly advancing technology and a phenomenon known as color constancy, have fostered acceptance of a rather broad range of unsaturated colors as "white". Color constancy refers to the well-known fact that the level and color of slightly unsaturated or near-white illumination over an area can vary moderately without substantially altering the perceived colors of objects in that setting relative to one another. An example of this is the appearance of an outdoor scene to an observer wearing slightly amber or green sunglasses. After a brief moment of adaptation upon donning the sunglasses, an observer becomes unaware that the scene is being passed through a slightly colored filter. Another example is the tacit acceptance of a wide variety of "white" illuminators in residential, commercial, and public illumination. The bluish or cool white from various fluorescent lamps is virtually universal in office buildings, whereas the yellowish or warn white of incandescent lamps is dominant in residential lighting. The brilliant bluish-white of mercury vapor and metal halide lamps is commonplace in factory assembly lines, whereas the bronze-white emission of the high pressure sodium lamp dominates highway overhead lighting in urban areas. Despite the discernible tint of each of these sources which would be evident if they were compared side by side, they are generally accepted as white illuminators because their emissions are close enough to an unsaturated white to substantially preserve relative color constancy in the objects they illuminate. In other words, they render objects in a manner that is relatively faithful to their apparent "true" colors under conditions of natural illumination.
There are limits to the adaptability of human color vision, however, and color constancy does not hold if highly chromatic illuminators are used or if the white illumination observed in a setting is altered by a strongly colored filter. A good example of this limitation can be experienced by peering through a deeply colored pair of novelty sunglasses. If these glasses are red, for instance, then it will be nearly impossible to discern a line of red ink on white paper, even though the line would stand out quite plainly in normal room illumination if the glasses are removed. Another illustration of this effect is the low-pressure sodium lamp used for certain outdoor urban illumination tasks. This type of lamp emits a highly saturated yellow light which makes detection and or identification of certain objects or printed images very difficult if not impossible, and, consequently their commercial use has been very limited. As will be discussed later, a similar problem arises from prior-art attempts to use high intensity red or amber light emitting diodes (LEDs) as illuminators since they, like the low-pressure sodium lamp, emit narrow-band radiation without regard for rendering quality.
In order to improve the effectiveness of white light illumination systems, various support structures are typically employed to contain the assembly and provide energy or fuel to the incorporated light source therein. Furthermore, these systems typically incorporate an assortment of optical components to direct, project, intensify, filter or diffuse the light they produce. A modern vehicle headlamp assembly, for instance, commonly includes sealed electrical connectors, sophisticated injection-molded lenses and molded, metal-coated reflectors which work in concert to collimate and distribute white light from an incandescent, halogen, or arc-discharge source. A backlight illuminator for an instrument panel in a vehicle or control booth typically contains elaborate light pipes or guides, light diffusers and extractors.
Of course, traditional white light sources which generate light directly by fuel combustion are no longer suitable for most vehicular, watercraft, aircraft, and portable and certain other applications where an open flame is unsafe or undesirable. These therefore have been almost universally superseded by electrically-powered, white light sources. Furthermore, many modern electric light sources are relatively inefficient, e.g., conventional tungsten incandescent lamps, or require high voltages to operate, e.g., fluorescent and the gas discharge lamps, and therefore aren't optimal for vehicular, portable, and other unique illuminators used where only limited power is available, only low voltage is available or where high voltage is unacceptable for safety reasons.
Because no viable alternatives have been available, however, illuminators for these overland vehicles, watercraft, aircraft and the other fields mentioned have used low-voltage incandescent white-light illuminators for quite some time to assist their operators, occupants, or other observers in low light level situations. In automobiles, trucks, vans and the like, white light illuminators are used as dome lights, map lights, vanity mirror lights, courtesy lights, headlamps, back-up lights and illuminators for the trunk and engine compartments and license plate. In such vehicles, white light illuminators are also used to backlight translucent screen-printed indicia such as those found in an instrument cluster panel, door panel, or heater and ventilation control panel. Similar uses of white light incandescent illuminators are found on motorcycles, bicycles, electric vehicles and other overland craft. In aircraft, white-light illuminators are used in the passenger compartment as reading lamps, to illuminate the floor and exits during boarding, disembarking, and emergencies, to illuminate portions of the cockpit, and to back-light or edge-light circuit breaker panels and control panels. In watercraft such as ships, boats and submarines, white-light illuminators are used to illuminate the bridge, the decks, cabins and engineering spaces. In portable and specialty lighting applications, low-voltage white light illuminators are used as hand-held, battery-powered flashlights, as helmet-mounted or head-mounted lamps for mountaineering or mining, as automatically-activated emergency lighting for commercial buildings, as task lighting in volatile environments, and as illuminators in a wide variety of other situations where extreme reliability, low voltage, efficiency and compactness are important.
These aforementioned white-light illuminators rely almost exclusively upon incandescent lamps as light sources because incandescent bulbs are inexpensive to produce in a wide variety of forms and, more importantly, they produce copious quantities of white light. Despite this, incandescent lamps possess a number of shortcomings which must be taken into account when designing an illuminator assembly.
Incandescent lamps are fragile and have a short life even in stable environments and consequently must be replaced frequently at great inconvenience, hazard, and/or expense. This need for replacement has complicated designs for all manner of illuminators, but especially for vehicles. For example, in U.S. Pat. No. 4,087,096, Skogler et al. disclose a carrier module for supporting lamps for illuminating a portion of a vehicle interior. The carrier module has a rigid body and a pair of mounting projections for removably mounting the carrier module in a rearview mirror. The design even has an opening specifically designed to allow insertion of a tool for releasing the module from the rearview mirror. This carrier module is an excellent example of the Herculean design efforts taken by mirror manufactures to ensure incandescent lamps can be easily removed and replaced by a vehicle owner.
In addition to their inherently short life, incandescent lamps are very susceptible to damage from mechanical shock and vibration. Automobiles experience severe shocks and significant vibration during driving conditions which can cause damage to incandescent lamps, particularly the filaments from which their light emissions originate. This is an especially severe problem for lamps mounted on or near the engine hood, trunk lid, passenger doors, exterior mirrors, and rear hatch or gate, all of which periodically generate tremendous shocks upon closing. Aircraft and portable illuminators experience similar environments, and therefore another source of white light would be highly beneficial to decrease the time and cost associated with replacing lamps therein on a regular interval.
Incandescent lamps can also be easily destroyed by exposure to liquid moisture due to the thermo-mechanical stress associated with contact between the hot glass bulb wall and the room-temperature fluid. Incandescent lamps are also easily damaged by flying stones and the like. Thus, it is very difficult to incorporate an incandescent light on an exterior mirror without going to extreme measures to protect the light bulb from shock, vibration, moisture and flying objects while still allowing for removal of the light fixture when it either bums out or is otherwise permanently damaged.
Incandescent lights also exhibit certain electrical characteristics which make them inherently difficult to incorporate in vehicles, such as an automobile. For instance, when an incandescent light source is first energized by a voltage source, there is an initial surge of current which flows into the filament. This inrush current, which is typically 12 to 20 times the normal operating current, limits the lifetime of the lamp thus further amplifying the need for an illuminator structure which allows for frequent replacement. Inrush current also necessitates unusual consideration when designing supporting electrical circuits which contain them. Fuses, relays, mechanical or electronic switches, wire harnesses, and connectors electrically connected to such lamps must be capable of repeatedly carrying this extreme transient.
In addition, the voltage-current (V-I) characteristic of incandescent lamps is notoriously non-linear, as are each of the relationships between light output and voltage, current, or power. The luminous intensity, color temperature, and service life of incandescent lamps varies exponentially as a function applied current or voltage. This sensitivity to power source variation makes electronic control of incandescent lamps a particularly difficult problem. They are further susceptible to significant reliability and field service life degradation when subjected continuously to DC electrical power, pulse-width modulated DC power, simple on/off switching of any sort, or any over-voltage conditions, however minor. Incandescent lamps also possess significant inductance which, when combined with their relatively high current load, complicates electronic switching and control greatly due to inductive resonant voltage transients. A typical square wave, DC pulse modulation circuit for a 0.5 amp, 12.8 volt incandescent lamp might produce brief transients as high as 30 volts, for instance, depending on the switching time, the lamp involved, and the inductance, capacitance, and resistance of the remainder of the circuit.
Incandescent lamps also suffer from poor efficiency in converting electrical power into radiated visible white light. Most of the electrical energy they consume is wasted in the form of heat energy while less than 7% of the energy they consume is typically radiated as visible light. This has severe negative consequences for vehicular, aerospace, watercraft, and portable illuminator applications where the amount of power available for lighting systems is limited. In these applications, electrical power is provided by batteries which are periodically recharged by a generator on a ship or aircraft, an alternator in an automobile, by solar cells in the case of some remote or aerospace applications, or are otherwise periodically replaced or recharged with an AC/DC adapter such as in the case of a flashlight. Because these mechanisms for restoring battery charge are inherently bulky, heavy, and/or expensive, it is severely detrimental for an illuminator to possess poor power-conversion efficiency in generating visible light. An acute example of the importance in illuminator efficiency is the electric vehicle. For electric bicycles, mopeds, motorcycles, automobiles, golf carts, or passenger or cargo transfer carts, white-light illuminators in the form of electric headlamps, backup lamps, etc. consume an unusually large portion of the vehicle's limited power budget; hence they would benefit greatest from high-efficiency white-light illuminators. If a more efficient white-light source was available, much less power would be required to energize the illuminator and more power would be available for other systems. Alternatively, the power savings from an improved illuminator would allow for improved power supplies and energy storage or energy replacement mechanisms.
Another result of poor efficiency associated with incandescent lamps is that they generate large amounts of heat for an equivalent amount of generated light as compared to other sources. This results in very high bulb-wall temperatures typically in excess of 250 degrees C. and large heat accumulations which must be dissipated properly by radiation, convection, or conduction to prevent damage or destruction to the illuminator support members, enclosure, optics or to other nearby vehicle components. This high heat signature of common incandescent light sources in illuminators has a particularly notable impact on the specialized reflector and lens designs and materials used to collimate and direct the light. Design efforts to dissipate the heat while retaining optical effectiveness further add requirements for space and weight to the illuminator assembly, a severe disadvantage for vehicular, watercraft, aircraft and portable applications which are inherently sensitive to weight and space requirements.
Portable illuminators such as hand-held flashlights and head-mounted lamps experience similar problems stemming from incandescent white-light sources and would derive the same benefits from an improved system.
Physical mechanisms for generating white-light radiation other than incandescence and pyroluminescence are available, including various gas discharges, electroluminescence, photoluminescence, cathodoluminescence, chemiluminescence and thermoluminescence. The output of sources using these phenomena can be tailored to meet the requirements of specific systems; however, they have had limited use in vehicular, watercraft, aircraft or portable illuminators because of a combination of low intensity, poor efficiency, high voltage requirements, limited environmental resilience, high weight, complexity, high cost, poor reliability, or short service life.
More recently, great interest has been shown in the use of electroluminescent semi-conductor devices such as light emitting diodes (LEDs) as the light source for illuminator systems. Due to their strong coloration and relatively low luminous output as compared to incandescent lamps, early generations of LEDs found most of their utility as display devices, e.g., on/off and matrix-addressed indicators, etc. These uses still dominate the LED market today, however recent advances in LED materials, design and manufacturing have resulted in significant increases in LED luminous efficacy and, in their most recent commercial forms, exhibit a higher luminous efficacy than incandescent lights. But even the latest LEDs emit highly-saturated, narrow-bandwidth, distinctively non-white light of various hues. As discussed above, white light in one of its various manifestations is essential for most illuminator systems.
Despite the inherent colorfulness of LEDs, they offer many potential advantages as compared to other conventional low voltage light sources for vehicles, watercraft, aircraft and portable illuminators. LEDs are highly shock resistant and therefore provide significant advantages over incandescent and fluorescent bulbs which can shatter when subjected to mechanical or thermal shock. LEDs possess operating lifetimes from 200,000 hours to 1,000,000 hours, as compared to the typical 1,000 to 2,000 hours for incandescent lamps or 5,000-10,000 hours for fluorescent lamps.
It has been known that the narrow-band spectral emissions of several saturated light sources having different apparent colors can be combined to produce an additive color mixture having an apparent color which is different than that of any of its constituents. The basics of additive color are evident, for instance, in the observation that white sunlight decomposes into its constituent spectra when refracted by a prism or dispersions of water droplets such as occurs in a typical rainbow. The visible white light of the sun can therefore be considered an additive color mixture of all of the hues associated with its radiation in the visible spectrum having wavelengths from 380 to 780 nanometers.
An important and common example of additive color mixtures is the technique used in most color display screens possessing a cathode ray tube (CRT) or a liquid crystal display (LCD) element. These displays consist of addressable arrays of pixels, each of which contains sub-pixels having the hues red, green and blue which can be energized alone or in combinations. In the case of the CRT, each sub-pixel is a dot of inorganic phosphor which can be excited via cathodoluminescence by a steered electron beam. In the case of the LCD, each sub-pixel is a dot of colored dye in registry with a switchable liquid crystal shutter, the combination of which acts as a reconfigurable filter for a backlight. The result in either of these cases is that a brightly colored red sub-pixel can be energized simultaneously with an adjacent bright green pixel in unresolvable proximity to the red in order to form the perceived color yellow. A similar combination of the green sub-pixel and a blue one will form the perceived color cyan. A similar combination of the red sub-pixel and a blue one will form the perceived color magenta. Energizing all three of the red, green, and blue sub-pixels within a pixel concurrently will yield the perceived color white, if the brightness of each sub-pixel is proportioned properly. The relative proportions of the brightness of each of these differently colored sub-pixels can further be actively manipulated in a wide variety of combinations resulting in a continuum of perceived colors nearly replicating all of the colors available within human color vision, including white. Unfortunately, while these types of displays may exhibit appreciable surface brightness, they are extremely bulky, expensive and complicated and do not project suitable amounts of illumination at a distance to be of use as effective illuminators. For example, even the brightest and largest television screen casts only a dim glow across a darkened room. The illumination level associated with this dim glow is barely sufficient for reading a newspaper and is completely inadequate to identify objects or colors in a detailed photograph. However, the capability of such an R-G-B display system to reproduce appreciably all of the colors available within human color vision is an excellent example of the important phenomenon known as metamerism, which will be discussed in greater detail hereinafter.
LEDs are available in various hues and it is known that the output of red, blue and green LEDs can be combined in a fashion similar to that used for a CRT in the proper proportions to produce a variety of perceived colors, including the perceived color white. For example, in U.S. Pat. No. 5,136,483, Karl-Heinz Schoniger et al. disclose a light emitting device having twelve LEDs arranged to form a headlamp or signaling lamp. Schoniger et al. also disclose that to produce white light, red, green and blue LEDs need to be used simultaneously. However, such a system is rather complicated and Schoniger et al. do not mention the inherent susceptibility of an R-G-B system to unacceptable variation due to significant variations in luminous output produced from one LED to another of the same type. Such LED variations cause errors in the relative proportions of the actual color mixture produced versus that desired and, coupled with high complexity and cost, render the system undesirable for most practical uses.
Consequently, it is desirable to provide a highly reliable, low-voltage, long-lived, LED illuminator capable of producing white light with sufficient luminous intensity to illuminate subjects of interest well enough to be seen and to have sufficient apparent color and contrast so as to be readily identifiable.